Data dependent jitter (DDJ) is, for example, an important performance characteristic of high speed digital interfaces that quantifies to what degree a transition time depends on the history of bits before that transition. A transition of a digital signal, also called edge of a digital signal, means a change of the value of a digital signal from logical 1 to logical 0 or vice versa. So, data dependent jitter (DDJ) measures the dependency of transition times on the history of preceding bits and, therefore, inherently presupposes the knowledge of the immediate bit history before a measured transition. Because time measurements are usually not faster than 100 Msa/s (mega sample per second) and relevant high speed interfaces run at 2.5 Gb/s (gigabit per second) and above, it is generally impossible to timestamp all transitions. As a consequence, time stamping alone does not provide the useful information about the bit history.
In some applications, the bit stream outputted by a device under test (DUT) is not known by the test system. This unknown bit stream often comprises a repetitive bit sequence. From this bit sequence only the number of bits of the bit sequence and/or the number of edges or transitions within the bit sequence is known. From this information, the number of equal bits, also called run length, before each transition may be determined for data depending jitter analysis.
For example, there are known methods that can be applied to PRBS (pseudo random bit sequence) with known generating polynomial and unknown bit alignment. These methods do not apply to unknown bit sequences.
Jitter analysis based on time interval measurements provides the run length implicitly but makes spectral jitter decomposition very difficult. Q-space tail-fitting algorithms may be used.
Two time-stampers instead of one that time-stamp two subsequent transitions provide the run-length information as the difference between the two time-stamps. However, this involves double the amount of hardware and also doubles the memory bandwidth requirements to store time-stamp results.
Additional hardware to count the bits before each transition would increase memory bandwidth requirements to store time-stamps.
In another context, namely mixed-signal test, coherent under-sampling gathers complete information about a repetitive analog waveform by taking uniformly spaced samples at an integer fraction of the intended sample rate.